1. Field of the Invention
The present invention relates to a damper valve for a hydraulic power steering device, and in particular to a damper valve for a hydraulic power steering device whose structure is simplified and whose manufacturing cost is reduced.
2. Description of the Related Art
A hydraulic power steering device used for a vehicle is conventionally provided with a damper valve in order to improve steering stability at high vehicle speeds and to prevent occurrence of shimmy phenomenon, kickback phenomenon or the like. One such damper valve is disclosed in FIG. 11 (refer to Japanese Patent No. 2729982 Official Gazette).
A damper valve 030 of this type is provided with a partition plate member (valve holding member) 032 formed in a perforated plate-shape, where opening/closing of through holes 035 of the partition plate member 032 through which hydraulic oil passes can be controlled by a valve spring plate 036 supported on an end face of the partition plate member 032 via a pin 041 or a valve plate 037 subjected to spring action. The partition plate member 032 is provided within a valve casing 030 fixedly, so as not to be lost. Also, the valve spring plate 036 or the valve plate 037 is fixedly mounted to the partition wall member 032 so as not to become separated therefrom.
The pin 041 is provided at a center of the partition plate member 032, and comprises a large diameter portion and a small diameter portion. The pin 041 is positioned to the partition plate member 032 at a step portion between the large diameter portion and the small diameter portion, and a coil compression spring 050 is interposed between a flange of an end portion of the large diameter portion and an annular plate portion of the valve plate 037 to press the valve plate 037 on one end of the partition plate member 032. The valve plate 037 has a cylindrical portion protruded from its annular plate portion, the cylindrical portion being guided in a sliding manner by the large diameter portion of the pin 041. Also, a rivet-shape end portion (bending portion) of the small diameter portion of the pin 041 presses the valve spring plate 036 on the other end face of the partition plate member 032 via an annular plate 038.
A bored oil passage 051 is formed in the through holes 035 of the partition plate member 032 so as to secure a remaining area through which hydraulic oil can pass even when the valve spring plate 036 and the valve plate 037 are positioned at positions where they dose the through holes 035. Reference numeral 031 denotes a casing of the damper valve 030 and reference numeral 031b denotes a communication hole formed in the casing 030.
As the conventional damper valve is configured in the above manner, when vibrations are transmitted to a power cylinder from a road wheel side (not shown) via a tie-rod, hydraulic oil in a circuit connecting an oil passage changing-over valve and the power cylinder tends to flow in the damper valve 030 in a direction of arrow in FIG. 11 to return back toward the oil passage changing-over valve. However, as spring force of the valve spring plate 036 is set larger than the biasing force of the compression spring 050 pressing the valve plate 037, passage of the through holes 035 is opened by the valve spring plate 036 only in a slight amount, so that oil is subjected to flow resistance from the passage when it flows in the passage, and kinetic energy of the hydraulic oil is consumed.
As a result, since vibrations transmitted to the power cylinder from the road wheel side are damped and they are prevented from being transmitted to the steering wheel via the oil passage changing-over valve, stability and comfort of steering operation are not degraded. On the contrary, when hydraulic oil flows in a direction opposite of the arrow in FIG. 11, the valve plate 037 opens the passageway of the through holes 035 to a relatively large extent so that hydraulic oil is not prevented from being supplied to the power cylinder.
In the conventional device, however, since the structure of the partition plate member 032 or the pin 041 becomes complicated, two valve plates must be prepared, as the valve spring plate 036 and the valve plate 037, and a coil compression spring 050 or the like is required. The structure of the damper valve becomes complicated, and assembling thereof deteriorates, so that manufacturing cost of the damper valve is increased.